1. Field of the Invention
The present invention relates to a semiconductor device and a method for manufacturing the semiconductor device.
2. Related Art
The insulating films in semiconductor devices have become thinner by the improved semiconductor techniques for producing smaller devices. In semiconductor production, SiO2 is an excellent insulating material, and has been used over a long period of time. In a gate insulating film, however, the film thickness of a SiO2 film has become as small as the size equivalent to a few atomic layers. As a result, it has become difficult in principle to restrain current leakage through such a thin insulating film. Instead of SiO2, a material having higher permittivity can be used as an insulating film, and the material can electrically serve as if it were a thin SiO2 film. Even if such a film is thicker than the few atomic layers in the case of a SiO2 film, the film is electrically equivalent to the SiO2 film. Accordingly, it is assumed that current leakage can be restrained with such a film.
In a flash memory or the like, there is an interelectrode insulating film that isolates the control gate from the floating gate. However, such an interelectrode insulating film is also expected to have higher permittivity, as the device size has become smaller.
In this trend, high-permittivity insulating films (high-k films) have been studied, and as of today, gate insulating films containing hafnium are considered to have the greatest potential. However, the permittivity of a gate insulating film containing hafnium is approximately 25 at a maximum. In practice, the composition ratio of hafnium is highly likely to be lower than that. Therefore, such a gate insulating film containing hafnium as a high-k film can achieve relative permittivity as low as 12.
If a tetragonal crystalline structure can be formed from zirconia (zirconium oxide) or hafnia (hafnium oxide) through the first principle calculation, higher relative permittivity might be achieved. This possibility is suggested in by G. -M. Ringanese, X. Gonze, G. Jun, K. Cho, and A. Pasquarello in Phys. Rev. B69, 184301 (2004) (hereinafter referred to as Reference 1), for example. To confirm the possibility through experiments, they have tried to form tetragonal crystalline structures by adding yttrium to zirconia, or hafnia (disclosed by H. Kita, K. Kyuno, and A. Toriumi, in Appl. Phys. Lett. 86, 102906 (2005)) (hereinafter referred to as Reference 2).
Further, increases in permittivity by adding silicon to hafnia are disclosed by I. Tomida, H. Kita, K. Kyuno, and A. Toriumi in Appl. Phys. Spring Lectures, 25P-V-3, 2006 (hereinafter referred to as Reference 3).
By the technique disclosed in Reference 2, however, a tetragonal crystalline structure has not successfully been produced to provide the highest relative permittivity, as can be seen from the X-ray diffraction profiles.
In Reference 2, they tried to increase the permittivity by using rare-earth elements or alkaline-earth elements that were rarely used in semiconductor processes. Since those elements are soluble with zirconia and hafnia, it was considered that the permittivity could be easily increased with those materials. However, in the real semiconductor manufacturing processes that thoroughly refuse contamination, it is not easy to predict all the side effects and adverse influence that might be caused by the introduction of a rare-earth element or an alkaline-earth element. Therefore, the costs for introducing rare-earth elements or alkaline-earth elements are predicted to be very high.
If the dielectric disclosed in Reference 3 is used as a gate insulating film in real LSI production, as described later, the mobility might be reduced due to the stress exerted on the channel region in direct contact with the gate insulating film, or lattice relaxation to reduce the relative permittivity to the original value might be caused in the gate insulating film, or the gate insulating film might break itself because of the stress. As a result, the device characteristics might be degraded.